The collegiate principle of the regal _interregnum_ and the use of the lot[604] had both disappeared; the agreement of the patrician senators took the form of the election (_creatio_)[605] of a single interrex (_prodere interregem_). This magistrate nominated his successor, as the consul nominated the dictator,[606] each succeeding interrex holding office for five days. There was no limit to the number that might be created, the interreges varying from the minimum of two to the known maximum of fourteen;[607] but there must be at least two, the first being incapable of holding the consular election, probably because he was regarded as having received the _auspicia_ irregularly. The qualifications for the interrex were, that he should be a Patrician[608] and a senator, and the instances seem to show that he was invariably chosen from the past holders of curule office.[609] The first interrex was no doubt guided by the wishes of the Senate, or of the _patres_, in the choice of his nominee, and the whole list may have been prepared before his appointment. With the creation of the highest regular patrician office, i.e. with the election of a single consul, the _interregnum_ necessarily came to an end and the interrex retired. The reappointment of a chief magistrate called the people into life again; and, as a rule, it perpetuated itself by perpetuating the magistracy. There was, indeed, one large section of the people which had a continuity of existence as a corporation—this was the _concilium_ of the Plebs with its presidents, the tribunes. From the year 287 this _concilium_ was an independent legislative sovereign, and nothing more clearly marks the theoretical dualism of popular sovereignty at Rome than the fact that one parliament could continue to exist while the other, the _comitia_ in its various shapes, was dormant. The division of executive, judicial, even of deliberative power, is not uncommon in governments of the mixed type; the division of unlimited legislative authority is rarer and nowhere so clearly marked as in Rome; for an act of parliament did not require the co-operation of the two assemblies—the separate _fiat_ of each had the force of law.[610] It is true that in practice this fundamental dualism was not acutely felt, for the individual elements of the Populus and the Plebs were to all intents and purposes the same. We may emphasise the practical similarity and the theoretical difference best by glancing at the two assemblies of the tribes. Except in elective matters they differed hardly at all in the sphere of their competence—each was a legislative and judicial assembly. But they were under the presidency of magistrates of different kinds, and this caused a slight difference in their constitution. When the tribunes of the Plebs summoned the people by tribes, the members of the few patrician families did not attend; when the consul or praetor summoned the people by tribes,